1. Field of the Invention
This invention relates generally to ceramic bodies having electrical characteristics suitable for use as substrates for electronic packaging applications. More particularly, the invention is directed to a method and apparatus for preventing damage to sintered aluminum nitride substrates resulting from undesirable sticking during flattening or weighted sintering of the substrates. The flattened ceramic AlN substrates are particularly useful for multilayer metal-ceramic based microelectronic packages.
2. Discussion of the Related Art
As compared to alumina, the commercially predominant electronic ceramic, aluminum nitride ceramics potentially possess superior characteristics for electronic packaging applications with respect to electrical insulation, high thermal conductivity (above 120 W/m-K), thermal expansion match to silicon devices, and low dielectric constant. Aluminum nitride substrates are potentially useful where high heat dissipation is required in a microelectronic package, such as in a multilayer metal-ceramic package for high power devices. Aluminum nitride ceramics for microelectronic applications must therefore be capable of accommodating metallized components, polymeric layers and heat generating, high power electronic devices.
Aluminum nitride ceramics are prepared from aluminum nitride powders. In order to achieve suitable properties, the AlN ceramic must achieve a certain density of at least about 90%, and preferably greater than or equal to about 95%. Aluminum nitride with no sintering additives cannot densify to high density due to its very high equilibrium vapor pressure at sintering temperatures (on the order of 1850.degree. C.). However, densification can be achieved at lower temperatures by the use of sintering aids.
Sintering aids liquefy at temperatures below the decomposition and pure compound sintering temperatures for the ceramic, and promote densification of the ceramic grains by i) a particle rearrangement process mediated by capillary forces between the wetting liquid and the solid particles, and thereafter, ii) a dissolution and precipitation process. In this process, solid is preferentially dissolved at regions of high curvature (small particles) and redeposited at regions of low curvature (large particles). In addition, solid is preferentially dissolved at regions of solid-solid contact and redeposited away from the contact areas. At the later stages of the liquid sintering cycle, microstructure is refined via grain growth and coalescence processes.
Different combinations of sintering aids provide various compounds in situ which melt at different temperatures. The temperatures at which sintering occurs has an effect on the progress of the different types of sintering processes, and thus the microstructure and the final properties of the sintered ceramic body. Sintering aids also function to increase thermal conductivity of the sintered aluminum nitride body by gettering oxygen from the aluminum nitride powder. Thus, an effective sintering additive must form a liquid at a low temperature capable of dissolving and reprecipitating aluminum nitride without oxidation of the aluminum nitride. Not every liquid at sintering temperature will be able to getter oxygen and densify the ceramic.
All commercially available aluminum nitride powders contain oxygen as an impurity. This oxygen primarily takes two forms in the powder, as an alumina coating on each of the powder particles, and as dissolved oxygen impurity within the crystalline lattice of the aluminum nitride particles. A minor amount will be tied up as an oxide of any metal impurities which may be present. At a given sintering temperature, only a certain amount of oxygen, primarily from surface alumina and secondarily from other sources, will be available for reaction (hereinafter "available oxygen").
Upon densification, the volume of the green body, and for multilayer structures the volume of the metal lamina contained in the green body, together with the linear dimensions of the body, decrease as a function of both the temperature experienced and the particular material involved. If the metal and ceramic shrink at different rates, this shrinkage mismatch leads to residual stresses between the different constituent materials in the sintered body and distorts the final shape of the body. In order to maintain the exacting geometric tolerances required by the electronic packaging industry for multilayer ceramic based packages, it is necessary that the ceramic and the metal sinter at approximately the same rate.
An example of sintering of aluminum nitride at particularly low temperatures to mediate problems associated with different sintering rates and thermal expansion mismatches between the ceramic and metal portions of a multilayer electronic package is disclosed in U.S. Pat. No. 5,541,145, entitled Low Temperature Sintering Route for Aluminum Nitride Ceramics, issued Jul. 30, 1996, and incorporated herein by reference. In particular, an aluminum nitride ceramic having desired properties suitable for electronic packaging applications is prepared from an aluminum nitride powder/sintering aid mixture. The sintering aid includes a liquid component formed from alumina, calcia and boria, and a non-vitreous component including an element or compound of a metal of Group IIa, IIIa, or the lanthanides (rare earth metal compounds). The sintering aid may also include a multi-component liquid composition capable of forming the above components upon melting and thereafter crystallizing upon reaction.
Sintering aids are used to form an effective sintering liquid needed to density the ceramic and to remove dissolved oxygen from the AlN lattice. The sintering aids may also contribute to the formation of an additional phase or additional phases within the AlN structure which include reaction products of the sintering aid(s), aluminum and oxygen. For instance, second phase compositions have been found in sintered aluminum nitride ceramic bodies following sintering, including the presence of a residual calcium-aluminate containing species or calcium containing component. The calcium containing second phase may include a pseudo-quaternary compound containing Ca, Y, Al, and O and have substitutional boron contained within the compound, the second phase being in contact with the aluminum nitride at a dihedral angle sufficient to provide a resistivity of at least 10.sup.8 ohm-cm. Aluminum nitride sintered bodies can have enhanced properties using appropriate sintering aid packages within the sintering system even at low maximum sintering temperature, such as 1550.degree. C.-1700.degree. C., as disclosed in U.S. Pat. No. 5,541,145. This temperature regime is suitable for the simultaneous sintering of multiple metal and ceramic layers known in the art as co-fired multilayer electronic packages. As used herein, the term "low temperature" as it relates to sintered AlN bodies means an AlN body which is sintered at temperatures (&lt;1700.degree. C.) which are comparatively lower than temperatures at which the body would typically be sintered.
Further in connection with the manufacture of multi-layer electronic substrate modules, a nonuniform metal loading on a single layer and/or the layer to layer metal loading variation can lead to camber development in sintered parts even when the metal/ceramic systems are matched in their sintering behavior. The camber increases rapidly as the metal and ceramic systems deviate in their sintering behavior. Flattening or flat firing is therefore a common step after sintering, where the sintered parts are heated to an appropriate temperature under a flattening weight. The parts are thus flattened according to desired and/or required flatness specifications.
One of the major yield detractors in the flattening process is the undesirable sticking of setters (i.e., flattening weights) to the ceramic modules and/or substrates being flattened. Sticking leads to parts distortion, breakage and cracking. For aluminum nitride ceramic modules, the problem of sticking is particularly severe. It has been observed that aluminum nitride modules stick to molybdenum (Mo) setters during flattening, leading to cracking, spalling and fracture of parts, in some instances.